What is the social construction of race talking about?

Social Construction is the elaboration of values, rules, norms, meanings and social symbols carried out by society, based on the individual and social practices of each one. Society is constantly redefining and renegotiating these issues.

To say that something is socially constructed is to emphasize its dependence on contingent aspects of our social existence. That is to say: it could not have existed if we had not built it; and we didn't need to have built it at all, at least not in its present form.